Nuclear magnetic resonance (NMR) can be used to determine various characteristics of porous subsurface formations and/or samples. NMR logging tools can be used downhole to obtain these fluid-filled pore system characteristics, which then can be used to assist in the determination of, for example, the presence, absence, location, mobility, and producibility of hydrocarbons in a given formation or sample.
Conventional NMR logging generally involves deploying in a wellbore an NMR instrument, which uses magnetic fields to generate and detect various radio frequency (RF) signals from nuclei in a formation or sample. Certain archetypal NMR techniques are described in U.S. Pat. No. 6,232,778, the entire disclosure of which is hereby incorporated by reference.
NMR measurements, in general, are accomplished by causing the magnetic moments of nuclei in a formation to precess about an axis. The axis about which the nuclei precess may be established by applying a strong, polarizing, static magnetic field B0 to the formation, such as through the use of permanent magnets. This field causes the proton spins to align in a direction parallel to the applied field. This is sometimes referred to as the creation of longitudinal magnetization, resulting in the nuclei being “polarized”. Polarization does not occur immediately, but instead grows in accordance with a spin-lattice relaxation time constant T1, and may take as long as several seconds to occur. After sufficient time, a thermal equilibrium polarization parallel to B0 is established.
Next, a series of RF pulses are produced so that an oscillating magnetic field, B1, is applied. The first RF pulse must be strong enough to rotate the magnetization from B0 substantially into the transverse plane (i.e., transverse magnetization). Additional RF pulses are applied to create a series of spin echoes which bear information about various properties of the sample of interest such as volumes of liquids, their relaxation times, and diffusion properties. The frequency of the RF pulses is chosen to excite specific nuclear spins of a particular region of the sample that is being investigated. The time sequences and phases of the RF pulses are chosen to illuminate particular properties of the sample.
Two time constants are associated with the relaxation processes of longitudinal and transverse magnetization: T1 and T2. The spin-lattice relaxation time (T1) is the time constant for longitudinal magnetization to return to its thermal equilibrium value in the static magnetic field. The spin-spin relaxation time (T2) is the time constant for the transverse magnetization to return to its thermal equilibrium value of zero. In addition, diffusion coefficients of the liquids in the investigated sample affect the relaxation process as well. Distributions of these relaxation times are inferred from the amplitudes of the spin echoes mentioned above by T1, T2, and diffusion inversions, which can provide both one-dimensional distributions of any of those quantities and/or multi-dimensional joint distributions of any subset of them.